The present invention relates to a station belonging to a communications network in the form of a ring. More particularly, it is applicable to a network where various stations are connected to one another in series by means of a transmission medium or carrier, forming the ring, and the data are transmitted in a predetermined direction in the form of symbols (sets of bits).
Communications or data transmission networks are constituted by a plurality of units, generally called data terminal equipment, abbreviated as DTE, or "station" for the sake of linguistic simplicity. These stations communicate among one another by way of a transmission carrier, also called a transmission medium. In a network, data are transmitted over the medium as a series of frames. A frame is structured and includes beginning and end messages, synchronization signals from which a clock is derived, the address of the transmitting station, the address of the receiving station, the length of the data, the useful data, and so forth.
Among the data transmission networks most often used at present are ring networks, where the transmission carriers may be either coaxial cables or pairs of telephone wires, or increasingly frequently, optical fibers.
Regardless of the type of transmission carrier used in the ring network, a station connected to such a network has three essential functionalities:
Frame repetition: As a function of certain rules, the frames received are re-transmitted, which allows a frame to reach its destination after having passed through several stations;
Frame destruction: After having made one complete tour of the ring, the frame is destroyed by its transmitters, which avoids endless rotation of the frame over the ring;
Frame transmission: To transmit frames, some mechanism that authorizes or prohibits frame transmission at a given instant must be defined.
The operation of a network is defined by a set of rules and arrangements that make up what is commonly known as a protocol. In ring networks (also known as ring topology networks), the access protocols most commonly employed at present use the concept of a token. The token is a right to transmit. At a given instant, there is only one token on the ring. Any station that captures the circulating token accordingly keeps the right to transmit frames, preventing the others from doing so, and in that case does not repeat any frames it might receive. The right to transmit, that is, the time during which a station holds the token, is limited by a set of rules specific to the various protocols that use a token (for example, the FDDI token, an optical fiber network defined by ANSI Committee X3T9-5). At the end of transmission, the station that had captured the token lets it go. It accordingly gives up its right to transmit, thus leaving open the possibility for other stations to capture the token.
Consequently, the characteristics of a token protocol are as follows:
At a given instant, at most one station can transmit frames, or in other words gain access to the ring;
in the event that there is no exchange of data over the ring, or in other words on the condition of a zero load on it, a station must await the passage of the token before it can gain access to the ring;
the propagation time of the token on the ring is lost. In fact, during the transmit of the token from one station to another, it is impossible for a station to gain access to the ring, since this access requires capture of the token.
The consequences of the above characteristics are as follows:
Let us consider, for example, a ring having a latency (the propagation time necessary for any datum to make the complete tour of the ring) on the order of 5 ms, which for an FDDI optical fiber network corresponds to 1000 km of fiber. It is assumed that a station captures the token, transmits the frame, and then releases the token. From that time, it must then wait 5 ms before it can have the token again, even if there is no traffic whatever on the ring. This demonstrates the essential flow of token rings; The latency of the ring directly controls the frequency of access to it.
Instead of using the token concept, the station according to the invention and the ring network constituted by a set of such stations have the following qualities:
More than one of the stations according to the invention can simultaneously gain access to the ring, without collision.
In the case of a zero load, a station can gain access to the ring continuously.
Under a heavy load, the passband is distributed among the various stations, by a scheme determined in advance. This sharing may be equitable or not, depending on the users of the stations.
The ring access time is predetermined; that is, it has a maximum predetermined value.